1. Field of the Invention.
This invention relates to a phase perturbation compensation system for compensating a phase-perturbed signal in a data modem.
The invention has a particular application to high-speed modems in switched carrier configurations, such as multi-point networks and half-duplex point-to-point configurations. It is desirable in such networks and configurations for training sequences, which are applied at the commencement of transmissions, to be as short as possible.
One problem which arises in modem communication systems is that the communication channel introduces various kinds of deterioration, including carrier phase jitter and carrier frequency offset. For instance, phase jitter introduced on transmission lines such as telephone lines used for data communication is caused by AC ripple in the DC power supply for oscillators used in carrier systems, by incomplete band filtering, and by imperfections in quantization in digital systems. Frequency offset on transmission lines such as telephone lines used for data communication is caused in FDM (Frequency Division Multiplex) systems, since on most long haul lines there is a slight difference in frequency between the modulating and the demodulating carriers. High values for phase jitter and frequency offset can occur, particularly on long haul telephone lines such as are used for half-duplex point-to-point communication.
In both multipoint and half-duplex point-to-point configurations successive transmissions generally start at random instants. Therefore, it is not possible to start in a phase jitter compensation circuit from the state with previously acquired values and to continue tracking.
2. Description of the Prior Art.
U.S. Pat. No. 4,320,526 discloses an adaptive phase jitter compensator which includes a filter section which receives a derived phase error signal and its own weighted sum output to provide the difference between them as noisy estimates of the actual phase via taps of a delay line. A multiplier receives each tap signal to form a product used to update the tap weights used in the filter section. An up-modulator modulates a carrier signal with the weighted sum output, the up-modulator output being applied to a look-up table to form components compatible with the received data signal. A complex multiplier multiplies these components and the equalized data signal to provide both demodulation and phase jitter compensation.